At the risk of posting to an old, dead thread, I will say no matter what the technology, you get what you pay for. Especially as SDHC cards become common for SLRs and drive the price down even further, watch out for cheap cards. Buy from a reputable retailer, and don't always but the cheapest card. Especially, watch out for fakes. If the deal is too good to be true, it probably is!
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AngerClownJan 21 '11 at 4:38

6 Answers
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Initially the reason was performance and capacity. A lot of the pro-level cameras went CF because you needed the performance and, usually because a pro shoots RAW, the capacity which the original SD couldn't match. Not to mention there was a point in time where SD was more expensive as a result of volume sales differences. SD is now cheaper thanks in a big way to the proliferation of small devices that used them such as point and shoot cameras, media players, etc.

Anyways, the speed/capacity issues largely evaporated with SDHC and will evaporate with SDXC. Then I think you will see a big move, in fact the shift is already happening without SDXC support in any camera. Canon's 1Ds as noted, but even Pentax stayed SDHC with the 645D medium format and that is miles away from a consumer camera. Also what we are starting to see is dual card support because SD cards are nice and small, that's a nice thing to have and impractical with CF.

And since this thread has now been resurrected, I'd like to add another reason that I'm surprised nobody has mentioned.

Pro cameras tend to use Compact Flash because pro photographers prefer the size of Compact Flash cards. They're bigger, easier to handle with gloves on, harder to lose. They're also perceived as being tougher due to the size. Whether or not they are is irrelevant.

When you're shooting day in and day out to earn a living these things really matter!

I guess in part because CF cards are bigger, and so it's easier to develop faster and larger quantities of memory in large-sizes, before scaling down to the smaller SD cards later on. I think the added bulk and resilience of the more substantial CF cards.

I'd imagine most of the professional camera body users also have a fair amount already invested in CF cards, and so are likely to be reluctant to have to replace all of their CF cards with SDs.

Some professional DSLRs do support SDHC, such as Canon's 1Ds Mark III. Historically, Compact Flash offered higher capacity with miniature hard disks, although with advances in flash memory, solid state media has long overtaken it. Manufacturers don't want to give any excuse for photographers from upgrading, so tend to carry on supporting CF.

As SD cards are getting faster and more common, they should gradually displace CF cards in the professional market.

The fastest commonly-available SD cards on the market can reach speeds of up to 100 MB/s, so performance is much less of an issue than it once was. With the introduction of UHS-II (and with Toshiba already having developed a 260 MB/s card), it's only a matter of time before more pro-level cameras adopt SD cards.

Most laptop computers now have a media reader that accepts SD cards, but CF cards usually require separate equipment to read. The vast majority of consumer cameras accept SD cards, so it's a lot more convenient for enthusiasts to be able to keep using their existing memory cards when they move up to a more powerful camera.

The real reason CF is still on the market is historical: CF was once widely used for both professional and consumer cameras. When SD cards were first introduced (as MMC), they were significantly smaller than CF cards making them ideal for compact cameras, but had technical limitations which significantly limited their speed and maximum capacity. CF cards communicate using the PATA (or EIDE) interface once widely used by hard drives and are physically much larger than SD cards, making it easier to design for high performance and capacity. They were therefore retained for use in professional cameras where compactness is not as important as speed and capacity. Now that the original technical limitations of the SD card interface have been overcome, SD cards should gradually replace CF cards in pro-level equipment.

This transition to SD in high-end equipment is already in progress. Enthusiast-class cameras like the Nikon D600 and D7100, Canon EOS 6D and 70D, and Pentax K-3 all take SD cards and have UHS-I support. Given that cameras in this class have traditionally used CF cards, we can expect this transition to continue into the semi-pro and professional classes.